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Synthesis, Structure, and Kinetics and Stereochemistry of Base-Catalyzed Hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, Bis(pentaamine) Complexes Devoid of Deprotonatable NH Centers W. Gregory Jackson,* Alistair J. Dickie, and Josephine A. McKeon School of Physical, EnVironmental and Mathematical Sciences, Chemistry, UniVersity College, The UniVersity of New South Wales, Australian Defence Force Academy, Canberra, ACT, Australia 2600 Leone Spiccia and Suzanne J. Brudenell Chemistry Department, Monash UniVersity, Clayton, Victoria, Australia 3168 David C. R. Hockless and Anthony C. Willis Research School of Chemistry, Australian National UniVersity, Canberra, ACT, Australia 0200 Received July 6, 2004
The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co2(tmpdtne)Cl2]4+ which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co2(tmpdtne)(OH)2]4+ with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl- ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl- via a hydroxobridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The 1H NMR spectra for the hydroxo ions show very high field Co−OH resonances (ca. δ −0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published 1H NMR data for bridged Co−OH−Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH-], and deprotonation at an R-CH2 center (R to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of kOH for the second-order base-catalyzed reaction are ca. 4.0 M-1 s-1 for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.
Introduction synthesis1
Spiccia et al have described the of the potentially dinucleating ligand tmpdtne derived from dtne (Figure 1) and described first by Takamoto2 and later Wieghardt3 and have reported on some of its metal ion chemistry.4-7 * Author to whom correspondence should be addressed. E-mail: wgj@ adfa.edu.au. (1) Brudenell, S. J.; Spiccia, L.; Tiekink, E. R. T. Inorg. Chem. 1996, 35, 1974.
10.1021/ic0400888 CCC: $30.25 Published on Web 12/16/2004
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A particular interest in this ligand developed from the fact that it was devoid of NH centers, and it provided an opportunity to demonstrate that its Co(III) complexes could (2) Tanaka, N.; Kobayashi, Y.; Takamoto, S. Chem. Lett. 1977, 107. (3) Wieghardt, K.; Tolksdorf, I.; Herrman, W. Inorg. Chem. 1985, 24, 1230. (4) Brudenell, S. J.; Spiccia, L.; Bond, A. M.; Mahon, P. C.; Hockless, D. C. R. J. Chem. Soc., Dalton Trans. 1998, 3919. (5) Spiccia, L.; Fallon, G. D.; Grannas, M. J.; Nichols, P. J.; Tiekink, E. R. T. Inorg. Chim. Acta 1998, 279, 192.
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Figure 1. Bis(pentadentate) and other pentadentate ligands. Scheme 1. Pseudoaminate Mechanism of Base Hydrolysis
hydrolyze without base catalysis. For cobalt(III) amine complexes, this has been an outstanding problem since amine complexes devoid of acidic NH centers have previously proved difficult if not impossible to synthesize.8 Our first attempt to examine this aspect of the normally accepted base-catalyzed hydrolysis mechanism (SN1CB) entailed the ligands dmptacn and dmpmetacn (Figure 1).9,10 The complex of each can exist in asym and sym forms (Figure 2), but only the asym isomers have been isolated thus far. We discovered that the complex of the dmpmetacn ligand base hydrolyzed by a pathway first order in [OH-].9 Moreover, the reaction was faster than that for its unsubstituted analogue, dmptacn, which bears a single NH center. These studies uncovered a new mechanism for base-catalyzed substitution, whereby deprotonation at a methylene center R to a pyridyl can generate an activating aminate ion, Scheme 1. This work provides further evidence for this unique mechanism and also examines the stereochemistry of the substitution process. In addition, it was of interest to see if the two pentaaminechlorocobalt(III) centers of the dinuclear complex of tmpdtne (Figure 3) behaved independently or otherwise in base-catalyzed hydrolysis; the expectation for remote and noninteracting cobalt centers was for sequential hydrolysis with the two rates related statistically, 2:1. (6) Brudenell, S. J.; Spiccia, L.; Bond, A. M.; Comba, P.; Hockless, D. C. R. Inorg. Chem. 1998, 37, 3705. (7) Brudenell, S. J.; Spiccia, L.; Bond, A. M.; Fallon, G. D.; Hockless, D. C. R.; Lazarev, G.; Mahon, P. J.; Tiekink, E. R. T. Inorg. Chem. 2000, 39, 881. (8) Jackson, W. G. Inorg. React. Mech. 2002, 4, 1. (9) Dickie, A. J.; Hockless, D. C. R.; Willis, A. C.; McKeon, J. A.; Jackson, W. G. Inorg. Chem. 2003, 42, 3822. (10) Jackson, W. G.; Bhula, R.; McKeon, J. A.; Brudenell, S. J.; Spiccia, L.; Hockless, D. C. R.; Willis, A. C. Inorg. Chem. 2004, 43, 6549.
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All chemicals were AnalaR or an equivalent grade. Carbon-13 and proton NMR spectra and were recorded on Varian XL-300 and Unity Plus 400 MHz instruments at 20 °C. Solvents used were D2O with dioxane as the internal reference (13C, δ 69.27 relative to DSS) and Me2SO-d6 with the central peak of the CD3 septet as the reference (13C, δ 39.37 relative to SiMe4). Visible/UV absorption spectra (300-600 nm) and absorbance-time traces were recorded on a HP8453 diode array spectrophotometer thermostated to 25.00 ( 0.05 °C with use of a Lauda RM6 circulating water bath. Infrared spectra were obtained for KBr disks on a Biorad FTIR instrument. Cation-exchange media used were Dowex 50Wx2 (H+ form, 200400 mesh; Biorad) and SP-Sephadex C25 (Na+ form; Pharmacia). Electrospray ionization mass spectra were recorded on either a Micromass Platform QMS with electrospray source or a Bruker BioApex 47e FTMS with a 4-7 T superconducting magnet and Analytica electrospray source. Carbon dioxide free Milli-Q water was used for all physical measurements. Syntheses. The free dtne base was made from the hydrobromide dtne‚6HBr‚2H2O.3 Subsequent reaction with 4 equiv of picolyl chloride provided tmpdtne as described.1 The dichlorocobalt(III) complex of the crude ligand was prepared by any of the three methods described for the mononuclear chloro complexes of dmpmetacn and dmptacn.9,10 The best is the reaction of free tmpdtne base with 2 equiv of trans-[Co(py)4Cl2]Cl‚6H2O11 in warm MeOH. The deep pink product was purified by ion exchange chromatography on a short Dowex column by elution with 4-5 M HCl. Crystallization from water using a fifth volume of HClO4 (70%) gave a quantitative precipitation of the very insoluble perchlorate salt. It was recrystallized from a concentrated solution in Me2SO by careful dilution with 1 M aqueous NaClO4. It was collected, washed with ethanol and ether, and air-dried. Anal. Calcd for Co2(C38H52N10)Cl2(ClO4)4‚2H2O: C, 35.9; H, 4.4; N, 11.0. Found: C, 35.7; H, 4.4; N, 10.8. Vis/UV (CH3CN): 370 572 M-1 cm-1 and 508 504 M-1 cm-1; these values are very similar to those for their mononuclear analogues, save for the factor of 2 in the values. They were used to determine the stoichiometry of other salts from their vis/UV spectra. Caution! Perchlorate salts of metal complexes containing organics are potentially explosiVe. They should be handled only in small quantities and neVer be heated in the solid state nor in solutions of neat organic solVents especially in the presence of concentrated HClO4. The quite soluble chloride salt was conveniently obtained from the purified perchlorate by adding some HCl (10 M) to a slurry of the salt in acetone/water (1:1) and then carefully diluting with excess acetone. Chromatography on a long (50 cm) Dowex column, again using HCl (4-5 M) as eluant, cleanly separated the material into two pink bands in comparable amounts. The rac-asym-asym-[Co2(tmpdtne)Cl2]4+ isomer elutes in front of the meso form; each was crystallized as chloride and perchlorate salts as described for the mixture. Tetrachlorozincate salts were also obtained, using “H2ZnCl4” (2 M ZnCl2 in 5 M HCl) as the precipitant. Equilibration of the meso- and rac-asym-asym-[Co2(tmpdtne)Cl2]Cl4 Complexes. A sample (0.1 g) of either the meso or rac species was dissolved in water (10 mL), and excess zinc dust (0.1 g) was added to the pink solution. On being stirred, the solution quickly decolorized. It was filtered and allowed to reoxidize in air; HCl was added (to the 5 M level), and the solution was heated (90 °C, 1 h). Chromatography on Dowex as above yielded two separate (11) Springborg, J.; Schaffer, C. E. Acta Chem. Scand. 1973, 27, 3312.
meso- and rac-[Co2(tmpdtne)Cl2]4+
Figure 2. Two geometric isomers for [Co(dmptacn)X]n+. Table 1. Crystal Data for rac-asym-asym-[Co2(tmpdtne)(OH)2](ClO4)4‚3H2O
Figure 3. Dinuclear chlorocobalt(III) complex of tmpdtne.
pink bands in comparable amounts which were isolated by crystallization as their perchlorate salts and shown to be the rac (band 1) and meso (band 2) complexes, respectively. [Co2(tmpdtne)(OH)2](ClO4)4‚5H2O. To a concentrated solution of the chloro derivative (meso or rac; chloride salt) in water (1.0 g/15 mL) was added 2 M NaOH (4 mL). Within 1 min hydrolysis was complete, and a fifth volume of NaClO4 (5 M) was added and the mixture set on ice. Deep red-pink crystals deposited in good yield (90%). The complexes were recrystallized from 0.01 M NaOH again using NaClO4 as precipitant. These were filtered out, washed with i-PrOH and ether, and air-dried. Anal. Calcd for Co2C38H54N10(OH)2(ClO4)4‚5H2O: C, 35.4; H, 5.0; N, 10.9; Cl, 11.0. Found: C, 35.2; H, 4.5; N, 10.6; Cl, 10.9. The rac isomer proved to be a trihydrate (X-ray structure). Steric Course of Base Hydrolysis. A sample (ca. 80 mg) of meso- or rac-asym-asym-[Co2(tmpdtne)Cl2]4+ (Cl- salt) was dissolved in D2O (0.8 mL) containing dioxane as an internal reference, and the 1H and 13C NMR spectra were recorded. Two drops of NaOD (10 M) were then added, and the NMR spectra were recorded for the hydroxo ion that was formed within seconds (established in control experiments). The product solution was acidified by addition of several drops of DCl (10 M) until the solution just became orange; protonation of the ligand does not involve rearrangement about the metal ion. The 1H and 13C NMR spectra were again recorded. Buffer Solutions. Buffers employed ethanolamine (0.40 M), partly neutralized (0.1-0.9 equiv) with HCl, with the ionic strength adjusted to 2.00 M with NaCl. They were diluted 1:1 for use. The p[H] determinations were performed as previously described.9 Base Hydrolysis Kinetics. The kinetics for the dichloro species were measured in a series of buffers at 25.00 ( 0.05 °C. NaCl rather than NaClO4 was used as the supporting electrolyte because of solubility limitations. All rates were measured as absorbance versus time traces in triplicate, using the in situ method (1 cm cell).9 Either the chloride or perchlorate salts (ca. 0.01 g) were used for all kinetic runs. These were carried out by thermally equilibrating equal volumes (1.00 mL) of each of a solution of the complex and the double-concentration buffer (2 M) in a bifurcated 1 cm cell and mixing rapidly to initiate reaction. Kinetic data were analyzed by weighted (1/σ2) nonlinear regression using Kaleidagraph running on a Macintosh or by importing the HP mkd files into Specfit and carrying out Global Spectral Analysis (SVD data reduction) to a first-order rate law.9
formula M cryst system space group a/Å b/Å c/Å β/deg V/Å3 Z T/K λ(Cu KR)/Å Dc/g cm-3 µ/cm-1 no. of obsd/unique data no. of refined params R Rw
C38H62C14Co2N10O21 1254.64 monoclinic P21/a 13.331(3) 23.353(5) 16.478(4) 94.98(2) 5111(2) 4 296.2 1.5418 1.631 77.8 8189/7586 (1630, I > 3σ(I)) 331 0.0901 0.0764
Crystallography. Orange crystals of rac-asym-asym-[Co2(tmpdtne)(OH)2](ClO4)4‚3H2O suitable for X-ray crystallographic analysis were grown by slow evaporation of an aqueous solution. An orange prismatic crystal having the approximate dimensions of 0.28 × 0.12 × 0.08 mm was mounted on a glass fiber. All measurements were made at 23 °C on a Rigaku AFC6R diffractometer with graphite-monochromated Cu KR radiation. Crystal data are given in Table 1. The structure was solved by direct methods.12 Owing to the lack of sufficiently intense reflection data, only Co and Cl atoms were refined with anisotropic displacement parameters and other non-H atoms were refined isotropically. H atoms attached to other atoms were included at geometrically determined positions and were not refined. All calculations were performed using the teXsan crystallographic software package of Molecular Structure Corp.13
Results and Discussion Synthesis and Characterization. The elution characteristics, microanalyses, and vis/UV spectra for the new dinuclear complexes were consistent with the formulation [Co2(tmpdtne)Cl2]Xn (Xn ) Cl4, (ClO4)4, or (ZnCl4)2). The microprobe and electrospray mass spectrum indicated a Co: Cl ratio of 1:3 for the perchlorate salt, while the IR spectrum showed peaks at 1090 and 625 cm-1 attributable to ClO4-, absorptions in the range 1400-1620 cm-1 assignable to CdC and CdN of pyridine, and a characteristic strong absorption at 3420 cm-1 due to lattice water. The 1H (Figure 4) and 13C (Figure 1S, Supporting Information) NMR spectra (12) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (13) teXsan. Single-Crystal Structure Analysis Software, version 1.7; Molecular Structure Corp.: 3200 Research Forest Drive, The Woodlands, TX 77381, 1992-1997.
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Figure 4. 1H NMR spectra (top to bottom) for asym-[Co(dmpmetacn)Cl]2+, asym-[Co(dmptacn)Cl]2+, and rac- and meso-asym-asym-[Co2(tmpdtne)Cl2]4+ in Me2SO-d6.
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meso- and rac-[Co2(tmpdtne)Cl2]4+
Figure 5. 13C NMR spectra for asym-asym-[Co2(tmpdtne)Cl2]4+ complexes in D2O (top) and Me2SO-d6 (bottom). The doubling in some peaks is due to the presence of two isomers of very similar configurations.
for each isomer were almost identical. The highest field peak in the 13C NMR spectra represents the bridging ethylene carbons, and the chemical shifts for other signals are clearly indicative of dinuclear cations that closely resemble those for asym-[Co(dmptacn)Cl]2+ and asym-[Co(dmpmetacn)Cl]2+ which are also shown. Moreover, the two asym “halves” of each dinuclear cation are symmetry related. Figure 5 shows the 13C spectra for the mixture prior to separation where partial doubling of the peaks first alerted us to the presence of two isomers. There are four possible geometric isomers for these dinuclear species: sym-sym, asym-sym, and asym-asym, the last of which has meso and racemic forms (Figure 6). The second and last isomers are, in principle, optically resolvable. The observation of 19 lines (19 × 2C) in the 13C NMR spectra eliminates the sym-sym form which should show only 10 lines (9 × 4C + 1 × 2C). Further, the sym-asym form is eliminated on the basis that this has no symmetry and should show 38 lines. Two halves of the molecule are reasonably remote such that there would be 18 lines for the asym “half” while the sym portion should show 18 lines also but in closely spaced doublets due to the chiral asym component rendering the 9 pairs of sym carbons diastereotopic. By the same argument, the spectra for the two asym-asym isomers (rac and meso) should be very similar, as indeed they are. The two halves are symmetry related (C2 or σ), and the spectra show just 19 lines each.
Figure 6. Two topologies possible for asym-asym-[Co2(tmpdtne)X2]n+.
Finally, we note that Zn reduction of either dichloro isomer yields the labile Co(II) species which after removal of the zinc undergoes ready aerial reoxidation to yield an ca.. 1:1 mixture of the rac and meso isomers, converted to the dichloro species using hot HCl. This proved to be a useful way of synthesizing one isomer from the other. Inorganic Chemistry, Vol. 44, No. 2, 2005
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Figure 7. 1H NMR spectra for the rac- (top) and meso- (bottom) asym-asym-[Co2(tmpdtne)(OH)2](ClO4)4‚xH2O complexes in Me2SO-d6, illustrating the same asym configuration about cobalt and the very high field Co-OH signals.
Stereochemistry of Substitution. Both the rac and meso complexes hydrolyzed through to the dihydroxo ions with complete retention. The intermediate hydroxochloro ions were not observed. The dihydroxo ions in hot HCl revert to the dichloro ions with complete retention. These results are consistent with those determined for the mononuclear analogues asym-[Co(dmpmetacn)Cl]2+ and asym-[Co(dmptacn)Cl]2+ where retention was established by using a regiospecifically D-labeled reactant for the former species9 and an optically resolved sample of the latter.10 The dihydroxo complexes derived from the respective dichloro complexes were isolated, and the X-ray structure of one of these species (vide infra) established that the faster eluting dichloro ion was the rac complex. The 1H (Figure 7) and 13C NMR spectra (Figure 2S, Supporting Information) are analogous to those of the dichloro ions, save for the very high field proton signals at ca. -0.5 ppm which are assigned to the Co-OH residue. Initially these were believed to be due to bridging OH, but they integrated for 2H and for a bis(pentaamine) species only a single OH bridge is possible. Very high field signals (
